The X-ray activity of the slowly rotating G giant δ CrB

Abstract

δ CrB is a single G giant whose distinctive characteristics include an X-ray luminosity exceptionally high for a slowly rotating star. δ CrB was observed in March 2003 by the XMM-Newton observatory. The X-ray spectra of δ CrB are described by a MEKAL plasma model with two components at 6.5 × 106 K and 107 K. Series of lines of highly ionized Fe and several lines of the Ly series are visible in RGS spectra, most notably from O and Ne. The oxygen abundance is similar to the average abundance of the other elements but the Ne/O ratio found for δ CrB seems higher than in the solar photosphere, reminiscent of a similar anomaly observed in a subset of solar flares and in active stellar coronae. The spectral fitting of the EPIC and RGS spectra of δ CrB suggests a corona configuration with little contribution from quiet regions similar to the Sun. On the contrary the temperature T ≈ 6.5 × 106 K of the "cool" plasma component is reminiscent of solar type active regions, while the hot (T ≈ 107 K) component may be caused by disruptions of magnetic fields associated with a permanent flaring activity. The analysis results of the XMM-Newton observation of δ CrB were compared with those of other single G giants with similar spectral type, mass and evolutionary status but with higher rotation rates. The comparison suggests that rapid rotation (P < 9 days) could increase the surface coverage with active regions and the flaring rate on G giants as expected from classical, helicity related, dynamo-driven activity. We argue that the X-ray emission of δ CrB and slowly rotating giants could be related to the existence of magnetic fields induced by turbulent motion.